1. Field of the Invention
This invention relates to an emission control device for an internal combustion engine, and specifically a technique for improving the efficiency of conversion of HC in an HC adsorption catalytic converter.
2. Description of the Related Art
Generally, in order to convert harmful substances (HC, CO, NOx, etc.) in exhaust to less harmful substances, a catalytic converter such as a three-way catalytic converter is provided in an exhaust passage of an engine (internal combustion engine).
The three-way catalytic converter does not fully exhibit its conversion performance until it reaches the catalyst activation temperature. Even when the three-way catalytic converter is disposed near the engine body for early activation of the catalyst, there remains a problem that HC emitted especially in large quantities in cold start of the engine cannot be converted satisfactorily. In order to solve this problem, an HC adsorption catalytic converter provided with an HC adsorbent effective in HC adsorption has been proposed.
Such HC adsorption catalytic converter has, however, a property that when reaching a fixed temperature (approximately 100° C. to 150° C.), it generally releases the HC adsorbed on the HC adsorbent, and this fixed temperature is lower than the activation temperature of the three-way catalyst (approximately 250° C. to 350° C.). Thus, the HC adsorption catalytic converter has a drawback that even though the three-way catalyst is provided among or downstream of the HC adsorbent, HC desorbed from the HC adsorbent is emitted without being converted before the three-way catalyst reaches the activation temperature.
In this view, there has been developed a device which, when the three-way catalyst has not reached the activation temperature yet, increases oxygen in exhaust by controlling the air-fuel ratio for the engine to be lean or stopping fuel supply to the engine (fuel cut), so as to remove the HC desorbed from the HC adsorbent by oxidation.
Further, particularly in a vehicle provided with an automatic transmission (A/T) connected with an engine by means of a fluid coupling, fuel cut in the cold start of the engine results in a deterioration in vehicle drivability. Thus, there has been developed a device which carries out fuel cut only for some of the cylinders (half of the cylinders, for example) and supplies fuel to the other cylinders (partial fuel cut), thereby removing the HC desorbed from the HC adsorbent by oxidation while preventing a decrease in engine output (see International Patent Publication No. WO2005/124130).
The above-mentioned configuration, in which the air-fuel mixture taken into the engine is controlled to produce exhaust with a lean air-fuel ratio while HC is being desorbed from the HC adsorbent of the HC adsorption catalytic converter, allows the HC desorbed to be removed by reaction with oxygen in the exhaust, but does not fully draw such effect. The same applies to the case in which partial fuel cut is performed.
Meanwhile, recent studies have confirmed that, while oxygen in the exhaust cannot react with the HC desorbed from the HC adsorbent satisfactorily as mentioned above, the oxygen once stored in the HC adsorption catalytic converter by its oxygen storage capacity (OSC) and then released has a high reactivity with the HC desorbed. This is thought to be because the oxygen once stored and then released by the OSC of the HC adsorption catalytic converter is in the form of a radical and therefore higher in reactivity compared with oxygen molecules in the atmosphere, and for this high reactivity, can satisfactorily oxidize the HC desorbed from the HC adsorbent of the HC adsorption catalytic converter.
In this case, however, how to cause the HC adsorption catalytic converter to once store oxygen and then release it is a problem.
In the configuration in which, for early activation, a three-way catalytic converter is disposed near the engine, upstream of the HC adsorption catalytic converter, it is not possible to cause the HC adsorption catalytic converter to release the oxygen stored therein by its OSC until the three-way catalytic converter releases all the oxygen stored therein. Hence, it takes time for the release of the oxygen stored in the HC adsorption catalytic converter to start, which results in low frequency of HC being oxidized satisfactorily.
In the case where the above-mentioned fuel cut is carried out during the deceleration of the vehicle, etc. so as to improve fuel economy, when the engine speed decreases to a certain level, recovery of fuel supply (recovery from fuel cut) is carried out so as to prevent the engine from stalling. Thus, it is conceivable to utilize this such that fuel cut performed while HC is being desorbed causes oxygen to be stored in the HC adsorption catalytic converter by its OSC, and then, recovery from the fuel cut causes the stored oxygen to be released to oxidize HC. This idea has, however, a problem that if, in order to cause the stored oxygen to be released, the exhaust air-fuel ratio is regulated to a highly rich level immediately after the recovery from the fuel cut by controlling the air-fuel mixture taken into the engine and such highly rich exhaust air-fuel ratio is maintained, a rapid increase in engine torque is caused so that vehicle drivability deteriorates after the recovery from the fuel cut. Conversely, if the level of richness of the exhaust air-fuel ratio immediately after the recovery from the fuel cut is not high enough, there are problems that it is difficult to cause the stored oxygen to be released, and that the oxygen remaining in the engine cylinders in large quantities easily produce NOx.